The present invention relates to a multilayer ceramic board and a method of manufacture of same. More particularly, the invention relates to a method of manufacture of a ceramic plate in which the conductive wiring pattern is formed via the layers and to the structure of such ceramic board.
When a device having the desired functions is manufactured, an electronic part such as, for example, a microminiature device, is mounted on a substrate and wiring is then provided. The substrate may comprise a multilayer substrate utilizing resin such as, for example, epoxy resin, as the main material, or utilizing ceramic material of Al.sub.2 O.sub.3 and MgO, etc, as the main material. Such substrates are currently well known. A substrate using ceramic material has excellent thermal characteristics such as heat proof characteristics, as compared with resin. Furthermore, ceramic has greater electrical insulative characteristics and greater electrical resistance than a substrate utilizing resin as the main material. For this reason, ceramic material is often used as a substrate or carrier for mounting electrical devices such as beam lead devices, microminiature devices, etc, which require minute wirings. The conventional multilayer ceramic board or carrier may be manufactured by the following process.
Ceramic powder such as alumina is subjected to milling together with a solvent to provide a slip. The slip is placed flat on a plastic film, using a doctor blade. The slip is then dried, thereby providing an unsintered ceramic plate or a green sheet. A pattern is printed on the green sheet according to a wiring pattern, by using electrically conductive paste in which electrically conductive powders such as Mo and W are mixed. At such time, a hole is made at a specified position on the green sheet, if necessary. The hole is then filled with the electrically conductive paste.
Several boards obtained by the aforedescribed process are laminated with the desired wiring patterns positioned mutually face to face. The wiring patterns are electrically connected via through holes, which are then filled with the electrically conductive paste as a conductor. The laminated green sheets are fired under optimum firing temperature for producing a dense ceramic.
When ceramic is used as the board material in the conventional method of manufacture, however, shrinkage of laminated green sheets occurs during the firing step. This influences the positioning accuracy of the wiring pattern of the board. Thus, for example, when firing shrinkage is kept within a deviation of .+-.0.5%, a displacement of about 250 microns or .mu. will occur at the edge of a board 100.times.100 mm, considering the origin of contraction at the center of the substrate. When the deviation is .+-.0.3%, the displacement is approximately 150.mu..
When the substrate is small in size, and the pattern density is low, and the pattern is formed with a clearance at least longer than the aforementioned "displacement", it is possible to alleviate pattern displacement due to shrinkage caused at the time of sintering by providing conductor land which is wider than the wiring pattern on the substrate in face to face arrangement with the hole and terminals. Therefore, no practical problem arises in mounting the elements. However, when the circuit substrate is of high density and the pattern clearance is 250.mu. or less, mounting of an element and connection with an external substrate become impossible due to displacement resulting from shrinkage.
In general, electrical connection between the layers of the multilayer ceramic board is provided via the through holes. The problem of displacement can be solved by making the land diameter of the through hole large. Practically, however, the substrate surface is covered primarily with elements mounted thereon or space for external connection terminals, so that wiring must often be provided between lands. For this reason, the land diameter is restricted by the pattern density. In other words, in a high density circuit substrate, deviation of shrinkage due to firing governs the density of the pattern, etc.
From the foregoing point of view, the conventional method of manufacture of a multilayer ceramic board may be said to have a drawback sufficient to prevent the preparation of a high density circuit substrate.
The principal object of the invention is to provide a method of manufacture of a multilayer ceramic board having fine wiring patterns on its surface.
An object of the invention is to provide a method of manufacture of a multilayer ceramic board utilizing few and simple steps.
Another object of the invention is to provide a method of manufacture of a multilayer ceramic board wherein two ceramic boards or circuit substrates are laminated and the continuity of patterns formed on each surface is established by simple steps.